It is often necessary to obtain information concerning the liquid contents within sealed or otherwise inaccessible articles. Inaccessibility might arise as a result of hazardous conditions such as radioactivity or toxic vapors in the vicinity of the article under investigation, or where a container is filled with highly toxic or highly flammable materials and it would be unsafe to make measurements that require direct contact between instruments and the container. Typically, simple properties of the liquid contents, such as the sound speed, density and how attenuating the liquid is, are sufficient to identify and characterize various sub-classes of chemicals as is demonstrated in U.S. Pat. No. 5,767,407 for "Noninvasive Identification Of Fluids By Swept-Frequency Acoustic Interferometry," which issued to Dipen N. Sinha on Jun. 16, 1998. However, this and other techniques require direct physical contact between the sensor and the item to be interrogated.
An assessment of liquid level in a container is also of significance. The only non-contact technique that is currently in use is a nuclear technique that essentially provides a x-ray image, but requires a radioactive source which generates its own safety issues.
Structural integrity of storage containers, pipes, reaction vessels and other mechanical structures is another area of importance for remote measurement technology. It would be of substantial value to be able to remotely identify and locate cracks and other mechanical defects in container walls and structural members (see, e.g., "Non-Destructive Analysis Of Defects Using Structural Wave Propagation" by M. Staudenmann and M. B. Sayir, (http://www.lfm.mavt.ethz.ch/.about.staudenm/eval-2.html) Jan. 31, 1995/HTML-Version by Markus Staudenmann, and "The Interaction Of Lamb Waves With Defects" by David N. Alleyne and Peter Cawley, IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control 39, 381 (1992)).
Often, ultrasonic nondestructive testing techniques require direct contact of an acoustic transduction device to excite the vibrational modes of the structure under evaluation. Likewise, direct contact of a receiving device is required to measure the response of the object. These requirements are reduced by immersing the structure in an efficient sound conducting fluid or by propagating an acoustic disturbance through a jet of fluid. Though these techniques are effective in a large number of applications, there are still many situations where direct contact or immersion are not feasible.
Remote excitation of low-frequency vibrations in mechanical structures and containers using linear sound producing devices such as speakers and ultrasonic air transducers is well known (see, e.g., "A High Precision Ultrasonic System For Vibration Measurements" by M. S. Young and Y. C. Li, Rev. Sci. Instrum. 63, 5435 (1992), "Noncontact Measurement Of Vibration Using Airborne Ultrasound" by Oliver Bou Matar et al., IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control 45, 626 (1998), "Laser Acoustic Spectroscopy For CW Verification" by Julio G. Rodriguez, Verification Technologies, First/Second Quarters (1992), p. 40, and "Nondestructive Evaluation (NDE) Tests On Chemical Weapons and Containers At Tooele Army Depot, DOE/ID-10346, July 1991, p. 16). However, because of the large wavelength of sound at low frequencies, the sound cannot be selectively directed at a individual target so that the distance between the excitation source and the object to be investigated must be small. Moreover, such devices have low bandwidth that restricts the frequency band of interrogation, and the generated sound emanates in all directions due to a lack of directivity of the sound generator, which represents a potential nuisance to surrounding areas and may actually be a hazard to the operator because of the high intensities required. Laser detection of vibrations may be employed without a mirror having to be attached to the object under investigation (see, e.g., "The Affordable Portable Laser Vibrometer" by Ometron, Inc. (http://www.Ometron.com and http://www.ImageAutomation.com/Ometron)).
Recent advances in noncontact nondestructive evaluation of materials include laser-generated sound and electromechanical acoustic transduction devices (EMATs). However, EMATs are required to be located only a short distance from an object under investigation (see, e.g., "High Frequency Resonant Electromagnetic Generation And Detection Of Ultrasonic Waves" by Katsuhrio Kawashima et al., Jpn. J. Appl. Phys. 33, 2837 (1994)), and laser-generated sound techniques require high optical power levels to generate detectable ultrasonic disturbances which render it problematic in situations where the material under investigation is unstable.
In "The Audio Spotlight: An Application Of Nonlinear Interaction Of Sound Waves To A New Type Of Loudspeaker Design" by Masahide Yoneyama et al., J. Acoust Soc. Am. 73, 1532 (1983), and in "Parametric Array In Air" by Mary Beth Bennett and David T. Blackstock, J. Acoust Soc. Am. 57, 562 (1975), an audio signal produced from an amplitude-modulated ultrasound primary wave radiated from a transducer array into air due to the self-demodulation effect from nonlinear mixing by the air, and the generation of a difference-frequency signal from the interaction of two collinear primary sound beams from a parametric array, respectively, are described. However, neither paper teaches the use of the low-frequency audio signal for analysis of structures or the contents thereof.
Accordingly it is an object of the present invention to provide an apparatus and method for nondestructively and remotely analyzing the integrity of structures and the contents of containers.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.